The invention relates to a paging device such as a wristwatch type receiver and/or transmitter and, in particular to a wrist-fitted wireless set having a conductive portion which makes contact with the human body when fitted over a user's wrist and having a structure for preventing the intrusion of static electricity.
Wristwatch type receivers, also known as fitted wireless sets, pagers, selective call receivers or paging devices, have been manufactured over the past few years with various structural improvements including a reduction in both size and weight. For example, it has been proposed to use a band antenna such that a receiver antenna is built into a wrist hand for the purpose of reducing the size of the receiver body. It has also been proposed to form the body case or back lid of the receiver with a conductive member or material. In this manner, the body case or back lid has been utilized as a circuit component for the purpose of reducing the number of parts to be interconnected within the receiver body. Such devices can be configured as transmitters and/or receiver/transmitter sets.
When the wristwatch type pager is fitted over a user's wrist, fastening fixture is coupled to a loop antenna and a back lid; make contact with the human body. Since the fastening fixture and the back lid are each formed of conductive material, current distribution applied through the conductors forming the antenna circuit may be affected and changed based on the contact of those conductive components with the human body. Changes in the current distribution lower performance parameters of the wireless set such as gain of the antenna and a Q value of the antenna circuit. Accordingly, the sensitivity of the antenna also declines. In this configuration, it is difficult to provide an antenna mechanism and circuit arrangement which can operate within the range of desired frequencies. In fact, to date, no effective measures have been proposed to deal with a decline of sensitivity caused by the contact of the wireless set with the human body.
The contact of the wireless set with the human body also causes a circuit breakdown in the wireless set by an intrusion of static electricity from the human body. Referring to FIG. 3, a circuit is employed in accordance with the prior art as a measure to deal with the intrusion of static electricity. In this circuit, a diode 8 is serially connected to an antenna 5 so that static electricity entering from the antenna side is discharged through diode 8 to ground. Therefore, the static electricity is prevented from entering the side of an input amplifying circuit 9 connected at a downstream stage to the antenna circuit. However, the addition of diode 8 increases the capacitance of the circuit which resonates with the antenna to form an antenna tuning circuit. The increase in capacitance may render the circuit incapable of tuning to a particular frequency at which the tuning should be intrinsically achieved. Thus, to date, there have been no proposed no measures for effectively preventing the intrusion of static electricity which causes a decline of sensitivity of the receiver.
It is, therefore, desirable to provide a wrist-fitted wireless set which prevents a decline of sensitivity of the receiver caused by the contact of the receiver with the human body and prevents an adverse intrusion of static electricity to the receiver.